In a PET measurement, the key is to detect the annihilation of electron-positron pairs in the sample and to localize the signal line on which the respective event has taken place. Based on a plurality of such events, it is then possible to determine the inner activity distribution of the sample using tomographic reconstruction.
During each event, two photons are emitted simultaneously in opposite directions, the spatial orientation thereof not being predictable, as this is isotropic. For this reason, a plurality of detectors are arranged around the sample in a PET scanner, thereby ensuring that each of the two photons resulting from an event strikes a detector. If two of the detectors, which generally are scintillation crystals having PMTs (photomultiplier tubes) or APDs (avalanche photodiodes), register an event simultaneously, which is monitored by a coincidence circuit, information is obtained to the effect that an event took place on a connecting line between the two detectors. This line is referred to as the signal line or line of response (LOR).
Previously, the measured coincidences of individual crystal combinations were associated with a certain signal line. Typically, a signal line is established by connecting a select central or representative point in the two crystals involved.
The disadvantage is that noise, and frequently also artifacts, are present in the recorded data. These interferences are decisively amplified by the tomographic reconstruction, which is equivalent to solving an inverse problem, and reduce the quality of the image that is ultimately obtained from the reconstruction.